The present invention relates to an oscillator circuit having an oscillator core with at least one inductance and, connected thereto, a first capacitance and a second capacitance, whose capacitance values are adjustable by a control voltage. A deattenuation amplifier is coupled to the oscillator core and has a first transistor and a second transistor, which are cross-coupled to one another in a non-direct-electrical coupling.
In applications in the mobile radio range, fully integrated voltage-controlled oscillators (VCO) having output frequencies in the gigahertz range are subject to increasingly stringent requirements with regard to their phase noise.
The principle of generating a sinusoidal oscillation which underlies LC oscillators is characterized by deattenuation of an LC tuned circuit with the aid of an amplifier, see for example, the reference by Tietze, and Schenk, titled xe2x80x9cHalbleiter-Schaltungstechnik [Semiconductor Circuitry]xe2x80x9d, 10th Edition 1993, page 458 et seq. Feedback differential amplifiers are usually used as amplifiers in integrated DCO circuits, and are constructed using CMOS or bipolar circuit technology. In this case, the feedback can be interpreted such that it forms a negative resistance that compensates a resistance loss in the resonant circuit or tuned circuit and thus deattenuates the resonant circuit.
The document titled xe2x80x9cA 1.24-GHz Monolithic CMOS VCO with Phase Noise of xe2x88x92137 dBc/Hz at a 3-Megahertz Offsetxe2x80x9d, by C. M. Hung et al., IEEE Microwave and Guided Wave Letters, Vol. 9, No. 3, March 1999, specifies a VCO which is constructed using CMOS circuit technology and has two transistors cross-coupled in a respective direct-electrical connection as amplifier. As the current source for the differential amplifier, a CMOS transistor connected as a current source is connected between the VCO core and the reference-ground potential terminal.
A VCO of the generic type, in which the transistors of the differential amplifier are cross-coupled in a non-direct-electrical coupling, is specified in the document titled xe2x80x9cA Fully Integrated VCO at 2 GHzxe2x80x9d, by M. Zannoth, B. Kolb, J. Fenk, and R. Weigl, IEEE Journal of Solid-State Circuits, Vol. 33, No. 12, December 1998. This is constructed using bipolar circuit technology.
The VCO circuits described have the disadvantage, however, that, on account of the current sources used, the voltage range available to the differential amplifier and hence the signal amplitudes at the resonator are limited, so that the phase noise properties that can be obtained are also limited.
It is accordingly an object of the invention to provide an oscillator circuit which overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which the modulation range is increased and the phase noise is reduced.
With the foregoing and other objects in view there is provided, in accordance with the invention, an oscillator circuit. The oscillator circuit contains an oscillator core having at least one inductance, a first capacitance connected to the inductance, and a second capacitance connected to the first capacitance. The first capacitance and the second capacitance have capacitance values adjustable by a control voltage. A reference-ground potential terminal is provided. A deattenuation amplifier is coupled to the oscillator core and has a first transistor and a second transistor cross-coupled to the first transistor in a non-direct-electrical coupling manner. The first transistor and the second transistor each have a first load terminal directly connected to the reference-ground potential terminal, a second load terminal, and a control terminal.
According to the invention, the object is achieved in the case of an oscillator circuit of the generic type in which a respective load terminal of a respective transistor is directly connected to a reference-ground potential terminal.
In this case, the load terminal of the transistors that is directly connected to the reference-ground potential terminal is preferably a source terminal of a field-effect transistor or an emitter terminal of a bipolar transistor.
In this case, the oscillator circuit is based on the principle of not connecting the transistors of the deattenuation amplifier to a current source which supplies a constant current, and consequently operating the cross-coupled transistors as a differential amplifier, but rather connecting the transistors directly to a reference-ground potential terminal, so that, in DC voltage turns, the entire supply voltage can be dropped across the transistors, and, consequently, a maximum voltage swing is possible and the transistors are operated as switches or current switches. In the cross-coupling of the transistors, this necessitates direct-electrical isolation between the control and the load terminals of the transistors in the cross-coupling. This can be achieved by non-direct-electrical couplings, such as a transformer coupling or a capacitive coupling.
Furthermore, a regulating circuit that regulates a bias voltage of the transistors and also an operating current of the circuit may be provided for the operation of the oscillator circuit.
Overall, the oscillator circuit exhibits high stability, good integrability, suitability for operation at high frequencies in the gigahertz range and a low phase noise on account of the higher modulation capability. On account of the higher voltage swing at the inputs of the deattenuation amplifier, it is possible to use transistors having a small configuration that can be operated with a higher current density. The use of smaller transistors that have smaller parasitic capacitances allows the use of larger capacitances with a tunable capacitance value in the oscillator core, thereby increasing the tuning range of the oscillator circuit.
Moreover, transistors with a higher current density have a better radio-frequency behavior, in particular a higher transition frequency.
In order to further increase the signal levels and also to achieve greater interference immunity, the oscillator circuit can be constructed using differential circuit technology. The oscillator circuit can be realized using CMOS circuit technology.
In an advantageous embodiment of the present invention, for the non-direct-electrical cross-coupling of the first and second transistors, a first transformer is provided, which inductively couples a load terminal of the first transistor to a control terminal of the second transistor, and a second transformer is provided, which inductively couples a load terminal of the second transistor to a control terminal of the first transistor. The transformer windings that form inductances may at the same time be inductances in the oscillator core of the LC oscillator. Instead of using two transformers, it is possible to use one transformer whose windings each have a center tap.
In an advantageous embodiment of the invention, the first and second transformers in each case have a turns ratio other than 1. In this case, the turns ratio of a transformer or inductive transformer is defined as the ratio of the number of turns of the primary and secondary windings. The transformer coupling thus makes it possible to increase the control voltages of the transistors relative to the voltages at the load terminal coupled to the control terminal, as a result of which the noise properties of the oscillator circuit are further improved.
In a preferred embodiment, for the non-direct-electrical cross-coupling of the first and second transistors, a respective coupling capacitor is connected between the control input of the first transistor and a load terminal of the second transistor and also between the control input of the second transistor and a load terminal of the first transistor. The coupling capacitors of the capacitive coupling can be integrated in a simple manner with a small area requirement. The capacitive coupling may additionally have coupling resistors for forming a bias coupling between load terminals of the transistors and a bias regulating circuit.
In a further advantageous embodiment, the regulating circuit for the bias voltage of the transistors is provided. The regulating circuit has a respective bias transistor which, on a control side, is in each case connected to the control input of the first and of the second transistor, respectively, and, on the load side, is coupled to the oscillator core and also connected to a reference current source. The bias voltage regulation of the transistors simultaneously holds the operating current of the oscillator circuit at a predetermined reference value. This results in the oscillator circuit being independent of fluctuations in the supply voltage and other varying parameters.
In a further preferred embodiment, the regulating circuit for the bias voltage of the transistors is provided which contains a bias transistor connected as a diode. The bias transistor is on the one hand connected to the reference-ground potential terminal and on the other hand is connected to a reference current source and coupled to the oscillator core. Besides saving a bias transistor, the configuration of the regulating circuit is suitable for realizing a lower-noise oscillator compared with the one mentioned above.
In a further preferred embodiment of the invention, output transistors are provided, whose control inputs are in each case connected to those of the first and of the second transistor, respectively, and which have load terminals at which an output signal of the oscillator circuit can be drawn off. The output signal is a differential signal. The current of the output signal is thus proportional to the current in the oscillator core, good synchronism properties being associated with this. For further signal processing, a cascode circuit, for example, can be connected to the output transistors.
In a further preferred embodiment, the first and second capacitances of the oscillator core in each case contain a capacitor with a variable-capacitance diode connected thereto. Either the capacitance values of the capacitors or the capacitance values of the variable-capacitance diodes or both capacitance values are adjustable by the control voltage that can be fed to the oscillator circuit.
In a further preferred embodiment of the present invention, the variable-capacitance diodes are directly connected to one another by their anodes. This further improves the integrability of the circuit.
In a further preferred embodiment of the invention, the transistors of the oscillator circuit are CMOS transistors.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an oscillator circuit, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.